1. Field of the Invention
The present invention relates to a spark plug having a center electrode which forms a spark discharge gap between itself and a ground electrode, and more particularly to a spark plug used for an internal combustion engine wherein a discharge gap portion of a center electrode is provided with a dissimilar metal material of noble metals or the like, and a method of producing the same.
2. Description of the Related Art
More energy saving of the internal combustion engine used for vehicles such as automobiles is demanded in view of environmental protection and effective use of natural sources. For instance, a low fuel consumption is demanded, and development has progressed in this direction. A high compression system as well as a lean mixture combustion system using leaner mixture apparatus is developed as a possible method therefor. To progress the leaner mixture, the sparking voltage in the spark discharge gap becomes large.
Such sparking voltage further increases because of the enlargement of the discharge gap due to the consumption of the electrode, and it is likely that the withstand voltage margin is reduced and a spark discharge at areas other than the discharge gap is caused, thereby inflicting unnecessary damage on the internal combustion engine. In order to avoid such problems, there is demand for a spark plug having an electrode portion forming a discharge gap which is provided with a dissimilar metal material of noble metals or the like with efficient consumption resistance.
In response to such demand, many spark plugs of a structure in which precious metal material is disposed in the discharge spark gap portion have been proposed (for example, Japanese examined patent publication Nos. 56-45265 and 62-31797).
In the Japanese examined patent publication No. 56-45265, a noble metal ring 252 is press-fitted to a small-diameter portion on the tip portion of the center electrode 251 as shown in FIG. 17a, then the center electrode 251 and the noble metal ring 252 are adhered and fixed to each other as shown in FIG. 17b.
In the Japanese examined patent publication No. 62-31797, as shown in FIG. 19A, a coaxial projection of reduced diameter is formed on a tip surface of a center electrode 261, a noble metal ring 262 and formed with a hole into which the projection fits is placed on a tip surface of the center portion, and this ring 262 is fixed to the tip surface of the center electrode 261 by welding. In this case, the outer diameter of the noble metal ring 262 which is secured by welding is formed by extruding with dies 263, and this noble metal ring 262 is deformed so as to surround the side outer surface on the tip portion of the center electrode 261.
According to the above structure of the spark plug which is produced simply by swaging after the noble metal ring 252 is press-fitted to the small-diameter portion, as the noble metal ring 252 is applied pressure innerwardly in the radial direction, the noble metal ring 252 is reduced in the diameter while being stretched in the axial direction. However, as an axial deformation of the top end portion 253 of the noble metal ring 252 is not restricted, the top end portion 253 is stretched greater than the other portion. Thus, the portion around the top end portion 53 forms a thin portion 254 as shown in FIG. 17B.
In case that such center electrode 251 is exposed to heating and cooling environment of the combustion chamber of the engine, as the center electrode 251 and the noble metal ring 252 have different coefficients of linear expansion, the thermal stress is caused due to the difference of the coefficients of the linear expansion. This thermal stress becomes remarkably large around the top end portion 54 of the noble metal ring 252 due to the thin thickness thereof, and the crack 255 is advanced as shown in FIG. 19. By this crack 255, the noble metal ring 252 is dislodged, or exfoliated, and there is a possibility that the gap may be bridged.
According to the structure of the spark plug which is produced by extruding after the noble metal ring is welded to the tip portion of the center electrode, the drawing and extruding by the dies 263 is proceeded with high friction between the surface of the center electrode and the surface of the dies. Accordingly, the surface of the noble metal ring 262 is largely stretched by the dies 263 with high friction. As a result, the bottom portion 263 of the noble metal ring 262 is formed in a extreme thinning shape. In case that the center electrode 261 is exposed to the heating and cooling environment of the combustion chamber of the engine, as described the above, the crack 264 is advanced from the bottom end portion 263 as shown in FIG. 20.
Especially, the noble metal ring 262 welded to the tip surface of the center electrode 261 is extruded in the producing, the noble metal ring 262 and the material forming the center electrode 261 in the center position have different ductilities. Consequently, difference of deformation resistances is caused, and in the case of axial stretching, it is impossible to maintain the thickness of the precious metal material in a uniform condition over the entire area, and further the dimension in the axial stretching lacks stability. This appears markedly when the contraction percentage (surface-reduction rate) become high. Consequently, the dimensions of the center electrode is restricted, and it becomes necessary that the noble metal portion of the side surface of the center electrode opposing the ground electrode is disposed over a wide area in view of the above problem.
Additionally, when producing a spark plug of such structure, the noble metal ring 262 is adhered to the center electrode 261 by welding and further extruding, and so the interface of the noble metal ring 262 and the center electrode 261 adhered thereto repeatedly sustains thermal stress due to temperature changes owing to combustion and to the difference in the coefficients of linear expansion owing to the dissimilarity of the materials.
In order to prevent fracture due to radial thermal stress, material thickness to ensure the thermal stress intensity which is acted upon becomes necessary, and it is also necessary to ensure the uniformity of the thickness in order to prevent the generation of concentrated stress.
When generating spark discharge in the spark plug, consumption occurs on the electrodes which form the discharge gap where the discharge is generated. The tip portion of the center electrode 261 opposing the ground electrode is most consumed, because the tip portion of the center electrode 261 is directly exposed to high temperature due to combustion, in addition to the spark consumption. Consequently, the tip portion of the electrode is transformed into a tapered shape in a short time. When such consumption due to spark discharge is taken into account as well, considerable thickness comes to be demanded for the noble metal material, which also detracts from economy.
Furthermore, stress oxidation due to radial stress advances on the interface of the center electrode 261 and noble metal ring 262. Consequently, there is a possibility that the noble metal ring 262 may be dislodged by thermal shock or vibration with long-term use and the gap may be dislodged.